About 6,000 out of 140,000 officers in England and Wales are trained in guns.

The Met Police spokesman said it may need help in some specialist areas of policing but no decision had been made.

Considerable expenditure

Sir Ian told the MPA committee meeting: "The principle must be that we don't want armed foreign police, but there's a 'but' - and the 'but' is twofold.

"One, you may not be able to get any foreign police unless they are armed because they won't feel at all easy being unarmed in public scenarios like that.

'But the second and I think most difficult one is the number of protected persons and whether we actually have, in the United Kingdom, sufficient capacity to actually have enough armed officers to do the job.

"If we don't have, then you are then talking about a training requirement of very considerable expenditure for a period of eight weeks."

The Met, Britain's biggest force, has 2,600 firearms-trained officers that are mainly used to protect key buildings, such as Parliament, Buckingham Palace and foreign embassies.

A spokesman for the Met said: "The Olympic Games are still five years away and the detail of our policing plan is still being worked on.

"Whilst we have made no formal request for mutual aid, as it is too soon to do so, it is anticipated that we may need support to fulfil some specialist areas of policing, of which firearms officers are a good example."